Thermoelectric materials have enormous potential in commercial, space and defense applications. Such applications include but are not limited to advanced household refrigeration systems, cooling units for cellular base stations, system controls for increasing the fuel efficiency of trucks and cars, computer chips, infrared detectors, recreational coolers, power generation systems aboard spacecraft, as well as air conditioning and power systems in submarines. The primary advantage characteristic of thermoelectric materials is that they are all solid state. They do not require refrigerants or lubricants and they have no moving parts so that they produce no vibrations, no emissions and no noise. As a result, they provide extremely desirable operating characteristics and are also extremely reliable in operation.
The majority of the thermoelectric materials currently in use were discovered and investigated from about 1955-1965. Among these are several Bi--Te--Se, Pb--Te, Bi--Sb alloys. More recent developments in thermoelectric materials include those disclosed in U.S. Pat. 5,415,699 to Harman, U.S. Pat. Nos. 5,108,515 and 5,246,504 to Ohta et al. and U.S. Pat. No. 4,929,282 to Brun et al.
Unfortunately, the efficiency of presently known thermoelectric materials and systems is not very high and, therefore, the principal uses of thermoelectric devices are in applications where reliability is more important than cost (eg. deep space exploration applications). While there are a limited number of thermoelectric materials available that have commercial applications at high temperatures (i.e. above 300.degree. K), very few thermoelectric materials known to those skilled in the art provide the necessary function for low temperature applications such as the construction of refrigerators for cooling a high-temperature superconductor from room temperature to its critical temperature T.sub.C (approximately 100.degree. K).
The overall performance of thermoelectric materials is determined by the figure of merit Z. This quantity is defined by the electrical (.sigma.) and thermal (.kappa.) conductivities as well as the thermoelectric power (or Seebeck coefficient) S of the material through the following relations: ##EQU1## where T is temperature and L is a constant. According to the Wiedemann-Franz law (eq. [3]), .sigma. is proportional to the electronic component of the thermal conductivity, .kappa..sub.el, while the lattice contribution, .kappa..sub.ph, is determined by the type of crystal lattice and the chemical position.
Equations [1-3] indicate that there are no straightforward ways to increase Z. Indeed, in metals, high .sigma. is usually accompanied by low S and high .kappa.. On the other hand, semiconducting materials exhibit high values of S and low .sigma.. Consequently, the search for thermoelectric materials has in the past to a large extent been conducted within the domain of narrow-band semiconductors: that is those which represent a compromise between the properties of metals and semiconductors and have reasonably high values for both S and .sigma.. Prior to the present invention, good thermoelectric materials exhibit Z of approximately 0.001 to 0.006 K.sup.-1 (eg., Z=approximately 0.003 K.sup.-1 at 300.degree. K for Bi.sub.2 Te.sub.3) and there are no good thermoelectric materials available that exhibit high Z values at temperatures below 200.degree. K.